very slow, crashing desktop

Hello.I just got my RPi and installed Raspbian a few days ago. I notice that the desktop system is very slow: every time I try to do something the CPU goes 100%, windows need like 10-20 seconds to open and the system eventually hangs if I open a few programs.Right now I had aMule and Transmission opened, and also Midori with a few tabs, and was barely manageable; after I added a torrent file and it begun downloading (to an external HDD) at 800 KB/s aprox. the CPU when 100% all the time and crashed. Am I making it work too much?My root partition is mounted in an external HDD, so I don't think is a problem with my class 4 SDHC.Could it be a power problem? Right now the RPi is getting only 500mA from a DUB-H7 hub, until a buy an Y-cable.Thanky very much.